Mercurial > hg > truffle
view src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.cpp @ 1294:7de45b5044c3
6932270: Allow Java's ELF symtab reader to use separate debuginfo files
Reviewed-by: never
Contributed-by: Andrew Haley <aph@redhat.com>
author | never |
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date | Tue, 09 Mar 2010 11:02:39 -0800 |
parents | 98cb887364d3 |
children | c18cbe5936b8 |
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/* * Copyright 2001-2008 Sun Microsystems, Inc. All Rights Reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, * CA 95054 USA or visit www.sun.com if you need additional information or * have any questions. * */ # include "incls/_precompiled.incl" # include "incls/_cardTableExtension.cpp.incl" // Checks an individual oop for missing precise marks. Mark // may be either dirty or newgen. class CheckForUnmarkedOops : public OopClosure { private: PSYoungGen* _young_gen; CardTableExtension* _card_table; HeapWord* _unmarked_addr; jbyte* _unmarked_card; protected: template <class T> void do_oop_work(T* p) { oop obj = oopDesc::load_decode_heap_oop_not_null(p); if (_young_gen->is_in_reserved(obj) && !_card_table->addr_is_marked_imprecise(p)) { // Don't overwrite the first missing card mark if (_unmarked_addr == NULL) { _unmarked_addr = (HeapWord*)p; _unmarked_card = _card_table->byte_for(p); } } } public: CheckForUnmarkedOops(PSYoungGen* young_gen, CardTableExtension* card_table) : _young_gen(young_gen), _card_table(card_table), _unmarked_addr(NULL) { } virtual void do_oop(oop* p) { CheckForUnmarkedOops::do_oop_work(p); } virtual void do_oop(narrowOop* p) { CheckForUnmarkedOops::do_oop_work(p); } bool has_unmarked_oop() { return _unmarked_addr != NULL; } }; // Checks all objects for the existance of some type of mark, // precise or imprecise, dirty or newgen. class CheckForUnmarkedObjects : public ObjectClosure { private: PSYoungGen* _young_gen; CardTableExtension* _card_table; public: CheckForUnmarkedObjects() { ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap(); assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity"); _young_gen = heap->young_gen(); _card_table = (CardTableExtension*)heap->barrier_set(); // No point in asserting barrier set type here. Need to make CardTableExtension // a unique barrier set type. } // Card marks are not precise. The current system can leave us with // a mismash of precise marks and beginning of object marks. This means // we test for missing precise marks first. If any are found, we don't // fail unless the object head is also unmarked. virtual void do_object(oop obj) { CheckForUnmarkedOops object_check(_young_gen, _card_table); obj->oop_iterate(&object_check); if (object_check.has_unmarked_oop()) { assert(_card_table->addr_is_marked_imprecise(obj), "Found unmarked young_gen object"); } } }; // Checks for precise marking of oops as newgen. class CheckForPreciseMarks : public OopClosure { private: PSYoungGen* _young_gen; CardTableExtension* _card_table; protected: template <class T> void do_oop_work(T* p) { oop obj = oopDesc::load_decode_heap_oop_not_null(p); if (_young_gen->is_in_reserved(obj)) { assert(_card_table->addr_is_marked_precise(p), "Found unmarked precise oop"); _card_table->set_card_newgen(p); } } public: CheckForPreciseMarks( PSYoungGen* young_gen, CardTableExtension* card_table ) : _young_gen(young_gen), _card_table(card_table) { } virtual void do_oop(oop* p) { CheckForPreciseMarks::do_oop_work(p); } virtual void do_oop(narrowOop* p) { CheckForPreciseMarks::do_oop_work(p); } }; // We get passed the space_top value to prevent us from traversing into // the old_gen promotion labs, which cannot be safely parsed. void CardTableExtension::scavenge_contents(ObjectStartArray* start_array, MutableSpace* sp, HeapWord* space_top, PSPromotionManager* pm) { assert(start_array != NULL && sp != NULL && pm != NULL, "Sanity"); assert(start_array->covered_region().contains(sp->used_region()), "ObjectStartArray does not cover space"); bool depth_first = pm->depth_first(); if (sp->not_empty()) { oop* sp_top = (oop*)space_top; oop* prev_top = NULL; jbyte* current_card = byte_for(sp->bottom()); jbyte* end_card = byte_for(sp_top - 1); // sp_top is exclusive // scan card marking array while (current_card <= end_card) { jbyte value = *current_card; // skip clean cards if (card_is_clean(value)) { current_card++; } else { // we found a non-clean card jbyte* first_nonclean_card = current_card++; oop* bottom = (oop*)addr_for(first_nonclean_card); // find object starting on card oop* bottom_obj = (oop*)start_array->object_start((HeapWord*)bottom); // bottom_obj = (oop*)start_array->object_start((HeapWord*)bottom); assert(bottom_obj <= bottom, "just checking"); // make sure we don't scan oops we already looked at if (bottom < prev_top) bottom = prev_top; // figure out when to stop scanning jbyte* first_clean_card; oop* top; bool restart_scanning; do { restart_scanning = false; // find a clean card while (current_card <= end_card) { value = *current_card; if (card_is_clean(value)) break; current_card++; } // check if we reached the end, if so we are done if (current_card >= end_card) { first_clean_card = end_card + 1; current_card++; top = sp_top; } else { // we have a clean card, find object starting on that card first_clean_card = current_card++; top = (oop*)addr_for(first_clean_card); oop* top_obj = (oop*)start_array->object_start((HeapWord*)top); // top_obj = (oop*)start_array->object_start((HeapWord*)top); assert(top_obj <= top, "just checking"); if (oop(top_obj)->is_objArray() || oop(top_obj)->is_typeArray()) { // an arrayOop is starting on the clean card - since we do exact store // checks for objArrays we are done } else { // otherwise, it is possible that the object starting on the clean card // spans the entire card, and that the store happened on a later card. // figure out where the object ends top = top_obj + oop(top_obj)->size(); jbyte* top_card = CardTableModRefBS::byte_for(top - 1); // top is exclusive if (top_card > first_clean_card) { // object ends a different card current_card = top_card + 1; if (card_is_clean(*top_card)) { // the ending card is clean, we are done first_clean_card = top_card; } else { // the ending card is not clean, continue scanning at start of do-while restart_scanning = true; } } else { // object ends on the clean card, we are done. assert(first_clean_card == top_card, "just checking"); } } } } while (restart_scanning); // we know which cards to scan, now clear them while (first_nonclean_card < first_clean_card) { *first_nonclean_card++ = clean_card; } // scan oops in objects // hoisted the if (depth_first) check out of the loop if (depth_first){ do { oop(bottom_obj)->push_contents(pm); bottom_obj += oop(bottom_obj)->size(); assert(bottom_obj <= sp_top, "just checking"); } while (bottom_obj < top); pm->drain_stacks_cond_depth(); } else { do { oop(bottom_obj)->copy_contents(pm); bottom_obj += oop(bottom_obj)->size(); assert(bottom_obj <= sp_top, "just checking"); } while (bottom_obj < top); } // remember top oop* scanned prev_top = top; } } } } void CardTableExtension::scavenge_contents_parallel(ObjectStartArray* start_array, MutableSpace* sp, HeapWord* space_top, PSPromotionManager* pm, uint stripe_number) { int ssize = 128; // Naked constant! Work unit = 64k. int dirty_card_count = 0; bool depth_first = pm->depth_first(); oop* sp_top = (oop*)space_top; jbyte* start_card = byte_for(sp->bottom()); jbyte* end_card = byte_for(sp_top - 1) + 1; oop* last_scanned = NULL; // Prevent scanning objects more than once for (jbyte* slice = start_card; slice < end_card; slice += ssize*ParallelGCThreads) { jbyte* worker_start_card = slice + stripe_number * ssize; if (worker_start_card >= end_card) return; // We're done. jbyte* worker_end_card = worker_start_card + ssize; if (worker_end_card > end_card) worker_end_card = end_card; // We do not want to scan objects more than once. In order to accomplish // this, we assert that any object with an object head inside our 'slice' // belongs to us. We may need to extend the range of scanned cards if the // last object continues into the next 'slice'. // // Note! ending cards are exclusive! HeapWord* slice_start = addr_for(worker_start_card); HeapWord* slice_end = MIN2((HeapWord*) sp_top, addr_for(worker_end_card)); // If there are not objects starting within the chunk, skip it. if (!start_array->object_starts_in_range(slice_start, slice_end)) { continue; } // Update our beginning addr HeapWord* first_object = start_array->object_start(slice_start); debug_only(oop* first_object_within_slice = (oop*) first_object;) if (first_object < slice_start) { last_scanned = (oop*)(first_object + oop(first_object)->size()); debug_only(first_object_within_slice = last_scanned;) worker_start_card = byte_for(last_scanned); } // Update the ending addr if (slice_end < (HeapWord*)sp_top) { // The subtraction is important! An object may start precisely at slice_end. HeapWord* last_object = start_array->object_start(slice_end - 1); slice_end = last_object + oop(last_object)->size(); // worker_end_card is exclusive, so bump it one past the end of last_object's // covered span. worker_end_card = byte_for(slice_end) + 1; if (worker_end_card > end_card) worker_end_card = end_card; } assert(slice_end <= (HeapWord*)sp_top, "Last object in slice crosses space boundary"); assert(is_valid_card_address(worker_start_card), "Invalid worker start card"); assert(is_valid_card_address(worker_end_card), "Invalid worker end card"); // Note that worker_start_card >= worker_end_card is legal, and happens when // an object spans an entire slice. assert(worker_start_card <= end_card, "worker start card beyond end card"); assert(worker_end_card <= end_card, "worker end card beyond end card"); jbyte* current_card = worker_start_card; while (current_card < worker_end_card) { // Find an unclean card. while (current_card < worker_end_card && card_is_clean(*current_card)) { current_card++; } jbyte* first_unclean_card = current_card; // Find the end of a run of contiguous unclean cards while (current_card < worker_end_card && !card_is_clean(*current_card)) { while (current_card < worker_end_card && !card_is_clean(*current_card)) { current_card++; } if (current_card < worker_end_card) { // Some objects may be large enough to span several cards. If such // an object has more than one dirty card, separated by a clean card, // we will attempt to scan it twice. The test against "last_scanned" // prevents the redundant object scan, but it does not prevent newly // marked cards from being cleaned. HeapWord* last_object_in_dirty_region = start_array->object_start(addr_for(current_card)-1); size_t size_of_last_object = oop(last_object_in_dirty_region)->size(); HeapWord* end_of_last_object = last_object_in_dirty_region + size_of_last_object; jbyte* ending_card_of_last_object = byte_for(end_of_last_object); assert(ending_card_of_last_object <= worker_end_card, "ending_card_of_last_object is greater than worker_end_card"); if (ending_card_of_last_object > current_card) { // This means the object spans the next complete card. // We need to bump the current_card to ending_card_of_last_object current_card = ending_card_of_last_object; } } } jbyte* following_clean_card = current_card; if (first_unclean_card < worker_end_card) { oop* p = (oop*) start_array->object_start(addr_for(first_unclean_card)); assert((HeapWord*)p <= addr_for(first_unclean_card), "checking"); // "p" should always be >= "last_scanned" because newly GC dirtied // cards are no longer scanned again (see comment at end // of loop on the increment of "current_card"). Test that // hypothesis before removing this code. // If this code is removed, deal with the first time through // the loop when the last_scanned is the object starting in // the previous slice. assert((p >= last_scanned) || (last_scanned == first_object_within_slice), "Should no longer be possible"); if (p < last_scanned) { // Avoid scanning more than once; this can happen because // newgen cards set by GC may a different set than the // originally dirty set p = last_scanned; } oop* to = (oop*)addr_for(following_clean_card); // Test slice_end first! if ((HeapWord*)to > slice_end) { to = (oop*)slice_end; } else if (to > sp_top) { to = sp_top; } // we know which cards to scan, now clear them if (first_unclean_card <= worker_start_card+1) first_unclean_card = worker_start_card+1; if (following_clean_card >= worker_end_card-1) following_clean_card = worker_end_card-1; while (first_unclean_card < following_clean_card) { *first_unclean_card++ = clean_card; } const int interval = PrefetchScanIntervalInBytes; // scan all objects in the range if (interval != 0) { // hoisted the if (depth_first) check out of the loop if (depth_first) { while (p < to) { Prefetch::write(p, interval); oop m = oop(p); assert(m->is_oop_or_null(), "check for header"); m->push_contents(pm); p += m->size(); } pm->drain_stacks_cond_depth(); } else { while (p < to) { Prefetch::write(p, interval); oop m = oop(p); assert(m->is_oop_or_null(), "check for header"); m->copy_contents(pm); p += m->size(); } } } else { // hoisted the if (depth_first) check out of the loop if (depth_first) { while (p < to) { oop m = oop(p); assert(m->is_oop_or_null(), "check for header"); m->push_contents(pm); p += m->size(); } pm->drain_stacks_cond_depth(); } else { while (p < to) { oop m = oop(p); assert(m->is_oop_or_null(), "check for header"); m->copy_contents(pm); p += m->size(); } } } last_scanned = p; } // "current_card" is still the "following_clean_card" or // the current_card is >= the worker_end_card so the // loop will not execute again. assert((current_card == following_clean_card) || (current_card >= worker_end_card), "current_card should only be incremented if it still equals " "following_clean_card"); // Increment current_card so that it is not processed again. // It may now be dirty because a old-to-young pointer was // found on it an updated. If it is now dirty, it cannot be // be safely cleaned in the next iteration. current_card++; } } } // This should be called before a scavenge. void CardTableExtension::verify_all_young_refs_imprecise() { CheckForUnmarkedObjects check; ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap(); assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity"); PSOldGen* old_gen = heap->old_gen(); PSPermGen* perm_gen = heap->perm_gen(); old_gen->object_iterate(&check); perm_gen->object_iterate(&check); } // This should be called immediately after a scavenge, before mutators resume. void CardTableExtension::verify_all_young_refs_precise() { ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap(); assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity"); PSOldGen* old_gen = heap->old_gen(); PSPermGen* perm_gen = heap->perm_gen(); CheckForPreciseMarks check(heap->young_gen(), (CardTableExtension*)heap->barrier_set()); old_gen->oop_iterate(&check); perm_gen->oop_iterate(&check); verify_all_young_refs_precise_helper(old_gen->object_space()->used_region()); verify_all_young_refs_precise_helper(perm_gen->object_space()->used_region()); } void CardTableExtension::verify_all_young_refs_precise_helper(MemRegion mr) { CardTableExtension* card_table = (CardTableExtension*)Universe::heap()->barrier_set(); // FIX ME ASSERT HERE jbyte* bot = card_table->byte_for(mr.start()); jbyte* top = card_table->byte_for(mr.end()); while(bot <= top) { assert(*bot == clean_card || *bot == verify_card, "Found unwanted or unknown card mark"); if (*bot == verify_card) *bot = youngergen_card; bot++; } } bool CardTableExtension::addr_is_marked_imprecise(void *addr) { jbyte* p = byte_for(addr); jbyte val = *p; if (card_is_dirty(val)) return true; if (card_is_newgen(val)) return true; if (card_is_clean(val)) return false; assert(false, "Found unhandled card mark type"); return false; } // Also includes verify_card bool CardTableExtension::addr_is_marked_precise(void *addr) { jbyte* p = byte_for(addr); jbyte val = *p; if (card_is_newgen(val)) return true; if (card_is_verify(val)) return true; if (card_is_clean(val)) return false; if (card_is_dirty(val)) return false; assert(false, "Found unhandled card mark type"); return false; } // Assumes that only the base or the end changes. This allows indentification // of the region that is being resized. The // CardTableModRefBS::resize_covered_region() is used for the normal case // where the covered regions are growing or shrinking at the high end. // The method resize_covered_region_by_end() is analogous to // CardTableModRefBS::resize_covered_region() but // for regions that grow or shrink at the low end. void CardTableExtension::resize_covered_region(MemRegion new_region) { for (int i = 0; i < _cur_covered_regions; i++) { if (_covered[i].start() == new_region.start()) { // Found a covered region with the same start as the // new region. The region is growing or shrinking // from the start of the region. resize_covered_region_by_start(new_region); return; } if (_covered[i].start() > new_region.start()) { break; } } int changed_region = -1; for (int j = 0; j < _cur_covered_regions; j++) { if (_covered[j].end() == new_region.end()) { changed_region = j; // This is a case where the covered region is growing or shrinking // at the start of the region. assert(changed_region != -1, "Don't expect to add a covered region"); assert(_covered[changed_region].byte_size() != new_region.byte_size(), "The sizes should be different here"); resize_covered_region_by_end(changed_region, new_region); return; } } // This should only be a new covered region (where no existing // covered region matches at the start or the end). assert(_cur_covered_regions < _max_covered_regions, "An existing region should have been found"); resize_covered_region_by_start(new_region); } void CardTableExtension::resize_covered_region_by_start(MemRegion new_region) { CardTableModRefBS::resize_covered_region(new_region); debug_only(verify_guard();) } void CardTableExtension::resize_covered_region_by_end(int changed_region, MemRegion new_region) { assert(SafepointSynchronize::is_at_safepoint(), "Only expect an expansion at the low end at a GC"); debug_only(verify_guard();) #ifdef ASSERT for (int k = 0; k < _cur_covered_regions; k++) { if (_covered[k].end() == new_region.end()) { assert(changed_region == k, "Changed region is incorrect"); break; } } #endif // Commit new or uncommit old pages, if necessary. resize_commit_uncommit(changed_region, new_region); // Update card table entries resize_update_card_table_entries(changed_region, new_region); // Set the new start of the committed region resize_update_committed_table(changed_region, new_region); // Update the covered region resize_update_covered_table(changed_region, new_region); if (TraceCardTableModRefBS) { int ind = changed_region; gclog_or_tty->print_cr("CardTableModRefBS::resize_covered_region: "); gclog_or_tty->print_cr(" " " _covered[%d].start(): " INTPTR_FORMAT " _covered[%d].last(): " INTPTR_FORMAT, ind, _covered[ind].start(), ind, _covered[ind].last()); gclog_or_tty->print_cr(" " " _committed[%d].start(): " INTPTR_FORMAT " _committed[%d].last(): " INTPTR_FORMAT, ind, _committed[ind].start(), ind, _committed[ind].last()); gclog_or_tty->print_cr(" " " byte_for(start): " INTPTR_FORMAT " byte_for(last): " INTPTR_FORMAT, byte_for(_covered[ind].start()), byte_for(_covered[ind].last())); gclog_or_tty->print_cr(" " " addr_for(start): " INTPTR_FORMAT " addr_for(last): " INTPTR_FORMAT, addr_for((jbyte*) _committed[ind].start()), addr_for((jbyte*) _committed[ind].last())); } debug_only(verify_guard();) } void CardTableExtension::resize_commit_uncommit(int changed_region, MemRegion new_region) { // Commit new or uncommit old pages, if necessary. MemRegion cur_committed = _committed[changed_region]; assert(_covered[changed_region].end() == new_region.end(), "The ends of the regions are expected to match"); // Extend the start of this _committed region to // to cover the start of any previous _committed region. // This forms overlapping regions, but never interior regions. HeapWord* min_prev_start = lowest_prev_committed_start(changed_region); if (min_prev_start < cur_committed.start()) { // Only really need to set start of "cur_committed" to // the new start (min_prev_start) but assertion checking code // below use cur_committed.end() so make it correct. MemRegion new_committed = MemRegion(min_prev_start, cur_committed.end()); cur_committed = new_committed; } #ifdef ASSERT ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap(); assert(cur_committed.start() == (HeapWord*) align_size_up((uintptr_t) cur_committed.start(), os::vm_page_size()), "Starts should have proper alignment"); #endif jbyte* new_start = byte_for(new_region.start()); // Round down because this is for the start address HeapWord* new_start_aligned = (HeapWord*)align_size_down((uintptr_t)new_start, os::vm_page_size()); // The guard page is always committed and should not be committed over. // This method is used in cases where the generation is growing toward // lower addresses but the guard region is still at the end of the // card table. That still makes sense when looking for writes // off the end of the card table. if (new_start_aligned < cur_committed.start()) { // Expand the committed region // // Case A // |+ guard +| // |+ cur committed +++++++++| // |+ new committed +++++++++++++++++| // // Case B // |+ guard +| // |+ cur committed +| // |+ new committed +++++++| // // These are not expected because the calculation of the // cur committed region and the new committed region // share the same end for the covered region. // Case C // |+ guard +| // |+ cur committed +| // |+ new committed +++++++++++++++++| // Case D // |+ guard +| // |+ cur committed +++++++++++| // |+ new committed +++++++| HeapWord* new_end_for_commit = MIN2(cur_committed.end(), _guard_region.start()); if(new_start_aligned < new_end_for_commit) { MemRegion new_committed = MemRegion(new_start_aligned, new_end_for_commit); if (!os::commit_memory((char*)new_committed.start(), new_committed.byte_size())) { vm_exit_out_of_memory(new_committed.byte_size(), "card table expansion"); } } } else if (new_start_aligned > cur_committed.start()) { // Shrink the committed region MemRegion uncommit_region = committed_unique_to_self(changed_region, MemRegion(cur_committed.start(), new_start_aligned)); if (!uncommit_region.is_empty()) { if (!os::uncommit_memory((char*)uncommit_region.start(), uncommit_region.byte_size())) { vm_exit_out_of_memory(uncommit_region.byte_size(), "card table contraction"); } } } assert(_committed[changed_region].end() == cur_committed.end(), "end should not change"); } void CardTableExtension::resize_update_committed_table(int changed_region, MemRegion new_region) { jbyte* new_start = byte_for(new_region.start()); // Set the new start of the committed region HeapWord* new_start_aligned = (HeapWord*)align_size_down((uintptr_t)new_start, os::vm_page_size()); MemRegion new_committed = MemRegion(new_start_aligned, _committed[changed_region].end()); _committed[changed_region] = new_committed; _committed[changed_region].set_start(new_start_aligned); } void CardTableExtension::resize_update_card_table_entries(int changed_region, MemRegion new_region) { debug_only(verify_guard();) MemRegion original_covered = _covered[changed_region]; // Initialize the card entries. Only consider the // region covered by the card table (_whole_heap) jbyte* entry; if (new_region.start() < _whole_heap.start()) { entry = byte_for(_whole_heap.start()); } else { entry = byte_for(new_region.start()); } jbyte* end = byte_for(original_covered.start()); // If _whole_heap starts at the original covered regions start, // this loop will not execute. while (entry < end) { *entry++ = clean_card; } } void CardTableExtension::resize_update_covered_table(int changed_region, MemRegion new_region) { // Update the covered region _covered[changed_region].set_start(new_region.start()); _covered[changed_region].set_word_size(new_region.word_size()); // reorder regions. There should only be at most 1 out // of order. for (int i = _cur_covered_regions-1 ; i > 0; i--) { if (_covered[i].start() < _covered[i-1].start()) { MemRegion covered_mr = _covered[i-1]; _covered[i-1] = _covered[i]; _covered[i] = covered_mr; MemRegion committed_mr = _committed[i-1]; _committed[i-1] = _committed[i]; _committed[i] = committed_mr; break; } } #ifdef ASSERT for (int m = 0; m < _cur_covered_regions-1; m++) { assert(_covered[m].start() <= _covered[m+1].start(), "Covered regions out of order"); assert(_committed[m].start() <= _committed[m+1].start(), "Committed regions out of order"); } #endif } // Returns the start of any committed region that is lower than // the target committed region (index ind) and that intersects the // target region. If none, return start of target region. // // ------------- // | | // ------------- // ------------ // | target | // ------------ // ------------- // | | // ------------- // ^ returns this // // ------------- // | | // ------------- // ------------ // | target | // ------------ // ------------- // | | // ------------- // ^ returns this HeapWord* CardTableExtension::lowest_prev_committed_start(int ind) const { assert(_cur_covered_regions >= 0, "Expecting at least on region"); HeapWord* min_start = _committed[ind].start(); for (int j = 0; j < ind; j++) { HeapWord* this_start = _committed[j].start(); if ((this_start < min_start) && !(_committed[j].intersection(_committed[ind])).is_empty()) { min_start = this_start; } } return min_start; }